Apparent Assimilation Efficiency Research Articles

Tissue-specific distribution and maternal transfer of polybrominated diphenyl ethers (PBDEs) and their metabolites in adult common sole (Solea solea L.) over an entire reproduction cycle

Tissue-specific accumulation and distribution of polybrominated diphenyl ethers (PBDEs) and their debrominated metabolites were studied in common sole (Solea solea) over an entire reproduction cycle. The fish were dietary-exposed to selected PBDEs in laboratory-controlled conditions for one year. Fish of both sexes were sampled throughout the reproduction cycle and their muscle, liver, viscera, carcass, skin and gonads (female) were analysed for total lipid content, PBDEs and their debrominated metabolites.On a wet weight basis, the concentrations of most spiked congeners showed an increase at the end of the exposure time in the whole body of fish of both sexes. Conversely, BDE-99 and BDE-209 – the two most highly metabolized congeners – did not show a similar increase, while their debrominated metabolites exhibited a linear increase over time.Biomagnification factors (BMFs) calculated in whole body were > 1 for all PBDEs except BDE-99 (0.89 ± 0.22) and BDE-209 (0.013 ± 0.006). BMFs were strongly correlated to apparent assimilation efficiencies, which ranged from 1.6% ± 0.7% (BDE-209) to 88% ± 11% (BDE-100).Fish carcass was the most predominant storage compartment for all PBDEs except BDE-209 in both male and female fish, followed by skin, muscle, female gonads, liver and visceral tissue. BDE-209 showed a different distribution and was stored more predominantly in the liver, viscera and female gonads than other congeners, probably due to its transport with lipoproteins. All PBDEs except BDE-209 showed equilibrium partitioning between the liver and other studied compartments, while BDE-209 showed a higher affinity with blood-enriched tissues, leading to higher liver / carcass, skin and muscle concentration ratios. Visceral tissue and liver exhibited the most pronounced differences in terms of concentration variations between sexes and over time. In females, both tissues showed a significant decrease in most PBDE concentrations (ww) and TLC in March during the spawning season, with low inter-individual variability, reflecting the striking utilisation of lipids by females for reproduction at this time. In males, no differences were found in concentrations in these compartments over time and a high inter-individual variability was observed.Maternal transfer of PBDEs from gonad to eggs was constant, regardless of spawn rank in the spawning season. Egg / gonad PBDE concentration ratios (in lw) were 1.3 ± 0.7 for all congeners and 2.6 ± 0.3 for BDE-209, suggesting a higher transfer of this congener from gonads to eggs.

Selection and intake of algal species in butterfish (Odax pullus; Labridae)

Whether animals increase or decrease food intake with decreasing diet quality, and how they respond to the offer of various diet items, is an active area of research. We tested specimens of the marine herbivorous butterfish (Odax pullus; Labridae) in a variety of no-choice, 2-choice and 4-choice experiments with various algal species. Algae differed in nitrogen and organic matter content as well as in apparent assimilation efficiency. Notably, nitrogen assimilation was related to organic matter assimilation at a slope lower than 1, suggesting an increasing contribution of microbial digestion in the more digestible diets. Food intake was lower, rather than higher, on the lowest quality algae, and higher on the highest quality algae, i.e. the fish did not follow a compensatory intake strategy, but a strategy described in mammals as ‘anticipatory’ where high-quality food is ingested in large amounts when available. Food intake was highest during 4-choice experiments, suggesting that mixing diet items, either to complement various nutrients or to avoid the accumulation of any particular toxin, is an important component of the nutritional strategy in O. pullus.

Digestive efficiencies of Cape white-eyes (Zosterops virens), red-winged starlings (Onychognathus morio) and speckled mousebirds (Colius striatus) fed varying concentrations of equicaloric glucose or sucrose artificial fruit diets

Digestive physiology is important for understanding the feeding behaviour of organisms. Specifically, studies on the digestive physiology of frugivorous and nectarivorous birds are important for elucidating their preference patterns in the wild and the selective pressures they exert on fruit pulp and nectar. In this study, digesta transit times and digestive efficiencies of three species of birds, the Cape white-eyes (Zosterops virens), red-winged starlings (Onychognathus morio) and speckled mousebirds (Colius striatus) were investigated on equicaloric glucose or sucrose artificial fruit diets. Three concentrations, approximating the natural range of sugar concentrations in sugary, bird-dispersed fruits were used: low (6.6%), medium (12.4%) and high (22%). Digesta transit times of birds increased with an increase in concentration for all diets but were generally higher on glucose diets. Intake rates, on the other hand, decreased with an increase in sugar concentration. All species of birds failed to maintain a constant assimilated energy intake on glucose diets but mousebirds and white-eyes maintained it on sucrose diets. Apparent assimilation efficiencies of glucose diets for all species were comparable and typical of those found in other frugivorous birds. However, assimilation efficiencies for sucrose diets differed widely with red-winged starlings displaying very low assimilation efficiencies and as a consequence; they lost significant body mass on all sucrose diets. These results demonstrate the importance of digestive physiology in explaining fruit selection patterns in frugivorous birds and how a seemingly trivial physiological trait can have dire ecological consequences.

Digestive parameters and energy assimilation of Cape porcupine on economically important crops

To better understand how Cape porcupines are able to successfully occupy many agricultural lands, we studied their digestive parameters and energy assimilation when fed three economically important agricultural crops (potatoes [Solanum tuberosum], sweet potatoes [Ipomoea batatas] and butternut [Juglans cinerea]). Daily food intake, daily gross energy intake, daily faecal energy loss, daily energy assimilated, daily water intake and apparent assimilation efficiency were calculated for each porcupine and for each diet trial. Cape porcupines maintained body mass on all three experimental diets and energy intake ranged from 3 002.36 kJ kg−1 d−1 (potato) to 4 499.00 kJ kg−1 d−1 (sweet potato). The moisture content in each experimental diet was relatively high (>60%) and preformed water intake of winter acclimated Cape porcupines ranged from 60 ml kg−1 d−1 (potato) to 100 ml kg−1 d−1 (sweet potato). Assimilation efficiencies were high (>85%) on all three diets. Cape porcupines ingested c. 1.5–2.7 kg of each crop daily, translating to potentially large economic impacts on farmers. Knowledge and understanding of Cape porcupine food intake and dietary requirements give insight to their potential impact on commercial crops. This may aid in developing management and conservation strategies needed to manage this human–wildlife conflict.

Effects of nicotine on the digestive performance of nectar-feeding birds reflect their relative tolerance to this alkaloid

The paradox of secondary metabolites, toxic defence compounds produced by plants, in nectar and fruits is well known. Deterrence of feeding by nectarivorous and frugivorous birds is better understood than the effect of these chemicals on the digestive performance of birds. Digestive parameters such as transit time and sugar assimilation are important in assessing nutrient utilization and deterrence may be related to post-ingestive effects involving these parameters. Nectar and many fruits contain mainly sugars and water, and avian consumers compensate for low sugar content in their diet by increasing food intake: this may also increase their intake of secondary metabolites. We investigated how the alkaloid nicotine, naturally present in nectar of Nicotiana species, influences compensatory feeding and digestive performance of nectar-feeding birds. High nicotine concentration negatively affected compensatory feeding and apparent assimilation efficiency of white-bellied sunbirds Cinnyris talatala and Cape white-eyes Zosterops virens; but nicotine slowed gut transit time only in the latter species. In contrast, food intake and digestive performance of dark-capped bulbuls Pycnonotus tricolor was unaffected by nicotine up to a concentration of 50μM. Bulbuls are primarily frugivorous; hence, they are more exposed to secondary metabolites than sunbirds and possibly white-eyes. Because their diet is richer in toxins, frugivorous birds may have evolved more efficient detoxification strategies than those of specialist nectar-feeding birds.

Assimilation efficiency of the Red-winged Starling Onychognathus morio fed Strelitzia nicolai arils

There is a paucity of studies investigating the digestive efficiencies of avian frugivores fed fibrous (also referred to as woolly) aril pulp. We fed whole Strelitzia nicolai seeds with arils to captive Red-winged Starlings (Onychognathus morio) to determine daily food and energy intake, digestive transit time and assimilation efficiency. Strelitzia nicolai arils were found to contain a remarkably high lipid content (66.78 %), with a correspondingly low water (7.61 %) and protein (5.79 %) content. Birds generally ate only the aril of S. nicolai seeds, but if they swallowed the seeds whole then they either defecated or regurgitated the seeds without the aril. Red-winged Starlings were found to have fast digestive transit times (approx. 24 min) and very high apparent assimilation efficiencies (approx. 96 %) of the aril of S. nicolai seeds.

Digestive efficiencies are independent of gut passage times in rainbow skinks (Trachylepis margaritifer)

Constraints on physiological processes imposed on ectotherms by environmental temperatures can be severe, affecting many aspects of their biology. Included in the suite of physiological processes affected is gut motility, with below optimum temperatures generally resulting in slow gut passage. Trachylepis margaritifer (rainbow skink) however presents an unusual pattern whereby gut passage time decreases at a low temperature compared to when at an intermediate temperature. It has been suggested that this may be a ‘cutting-your-losses’ response whereby nutritional gain is sacrificed by voiding the digesta to reduce the risk of these rotting within the gut at these low temperatures, and if this is so, it should result in reduced digestive performance at 25°C. We tested this hypothesis by measuring appetite, apparent digestive efficiency (ADE) and apparent assimilation efficiency (AAE) in T. margaritifer. We found that although temperature significantly affected appetite and gut passage time, it did not affect digestive efficiency. Both ADE (>90%) and AAE (>80%) were high and temperature-independent across the range tested. Thus, the ‘cutting-your-losses’ hypothesis does not explain faster gut passage at 25°C. High digestive parameters could be maintained by increasing concentrations of digestive enzyme at low temperatures but remains to be tested in this species.

Mistletoebirds and xylose: Australian frugivores differ in their handling of dietary sugars.

Carbohydrate-rich mistletoe fruits are consumed by a wide range of avian species. Small birds absorb a large portion of water-soluble nutrients, such as glucose, via the paracellular pathway. d-xylose, a pentose monosaccharide, is abundant in some nectars and mistletoe fruits consumed by birds, and it has been suggested that it is most likely absorbed via the paracellular pathway in birds. We measured apparent assimilation efficiency ([Formula: see text]) and bioavailability (f) for d-xylose and d- and l-glucose in three frugivorous Australian bird species. Mistletoebirds, silvereyes, and singing honeyeaters showed significantly lower [Formula: see text] for d-xylose than for d-glucose. Across two diet sugar concentrations, silvereyes and singing honeyeaters significantly increased f of both l-glucose (a metabolically inert isomer of d-glucose commonly used to quantify paracellular uptake) and d-xylose on the more concentrated diet, probably because of increased gut processing time. By contrast, mistletoebirds (mistletoe fruit specialists) did not vary f of either sugar with diet concentration. Mistletoebirds also showed higher f for d-xylose than l-glucose and eliminated d-xylose more slowly than silvereyes and singing honeyeaters, demonstrating differences in the handling of dietary xylose between these species. Our results suggest that d-xylose may be absorbed by both mediated and nonmediated mechanisms in mistletoebirds.

Assimilation efficiency and gut passage time in an African elapid snake, Hemachatus haemachatus

We measured apparent assimilation efficiency (AAE) and gut passage time in the African elapid snake Hemachatus haemachatus under laboratory conditions. AAE was dependent on food type, being 89.8% when snakes were fed frogs and 82.8% when fed mice. Differences in AAE for different meal types could be ascribed to the indigestible hair in mouse meals because differences were lost once this component of the mouse meals was removed from the calculation. AAE did not depend on snake body mass and there was no significant difference between AAE measures made at 20°C and 27°C for mouse meals. Gut passage time was 25% faster at 27°C than at 20°C, but was not affected by food type or snake body mass. Larger meals took longer to digest. Thus, H. haemachatus appears to offset reduced digestive performance at lower temperatures by retaining food in the gut for longer when at lower body temperatures (Tbs). These results fit an emerging pattern in snakes whereby digestive efficiency is generally high and insensitive to Tb over the selected thermal range, but may become dependent at lower Tbs when digestion is eventually arrested. The rate of digestion is highly temperature-sensitive and is fastest in the selected Tb range. Diet appears to affect digestive efficiency owing, mainly, to the presence of indigestible components such as hair. Using a comparative framework, venom does not appear to improve digestion for this species, at least over the temperature range tested, relative to other snake species in the literature.

Sugar assimilation and digestive efficiency in Wahlberg's epauletted fruit bat ( Epomophorus wahlbergi)

Fruit- and nectar-feeding bats have high energy demands because of the cost of flight, and sugar is a good fuel because it is easily digested and absorbed. This study investigated the digestive efficiency of different sugars at different concentrations in Wahlberg's epauletted fruit bat ( Epomophorus wahlbergi). We predicted that the sugar type and concentration would affect the total amount of solution consumed, while the total energy gained and the apparent assimilation efficiency would be high, irrespective of sugar type or concentration. Equicaloric solutions of two sugar types, glucose and sucrose, at low (10%), medium (15%) and high (25%) concentrations were offered in separate trials to bats. Total amount of solution consumed, total energy gained from each solution, and apparent assimilation efficiency, were measured. Bats had higher total volumetric intake of glucose and sucrose at the low concentrations than at the higher concentrations. However, bats maintained similar total energy intake on the respective glucose and sucrose concentrations. Bats were found to have high assimilation efficiencies on both glucose and sucrose irrespective of concentration. As bats used both sugars efficiently to maximize and maintain energy gain, it is expected that they feed opportunistically on fruit in the wild depending on temporal and spatial availability to obtain their energy requirements. Furthermore, fruit with high sucrose or glucose content will be consumed.

The ability of the deposit-feeding sea cucumber Australostichopus mollis to use natural variation in the biodeposits beneath mussel farms

The feeding biology and ecology of juveniles of the Australasian sea cucumber, Australostichopus mollis are not well understood. A better understanding may provide useful information for the development of the aquaculture of this species. Currently, A. mollis is being co-cultured beneath mussel farms and it is not clear if it can effectively use the wide natural variation in the food availability typically found under mussel farms. In this study the feeding and growth of juvenile A. mollis, was examined on feeds containing different levels of mussel, Perna canaliculus, biodeposits, resulting in different levels of total organic matter (TOM) (1, 4, 12, and 20%) covering a range similar to that found beneath a typical mussel farm in New Zealand. As the TOM content increased in the food from 4% to 20%, the ingestion rate of sea cucumbers decreased and both the apparent assimilation and food conversion efficiencies increased. There was no significant growth of sea cucumbers in the 1% TOM feed treatment, while sea cucumbers in the remaining treatments had similar final wet weights with a combined mean daily specific growth rate of 0.6% d − 1 . These results demonstrate the ability of this species to use different levels of TOM to generate similar growth rates mainly by changes in their feeding behaviour and digestive physiology. These changes in their feeding biology together with the spatial and temporal variation of food availability in the sediments impacted by the mussel farm need to be taken into account when selecting suitable mussel farms for co-culture, as well as when defining the initial stocking biomass of sea cucumbers.

Food intake rates, assimilation efficiency, and transit times of Knysna (Tauraco corythaix) Turacos fed South African indigenous fruit

Fleshy-fruited plants and frugivores share a mutualistic relationship with plants offering the frugivores a reward of fleshy fruit pulp in order to disperse their seeds. However, all fruits are not equal in terms of energy available to frugivores. Consequently, we measured the assimilation efficiencies of fruits of six indigenous tree species for an avian frugivore, the Knysna Turaco (Tauraco corythaix). We predicted that the Turacos would process these indigenous fruits efficiently and have fast transit rates and high intake rates irrespective of species. Fruits were fed to the Turacos in separate trials to determine daily food and energy intake, assimilation efficiencies and digestive transit times. Digestive transit times of Knysna Turacos (c. 12–28 min) fall within the range of published examples for other frugivorous birds. Apparent assimilation efficiencies of the fruit ranged from c. 15–84% and were generally lower than those observed for other avian frugivores. Intake rates (in terms of wet weight) varied among the fruit species, and Knysna Turacos had greater intake rates on fruit which they are able to assimilate efficiently. Future studies looking at the nutritional composition of indigenous forest fruit may provide insight into the Knysna Turacos assimilation efficiency and consequently their food preferences and role as potential seed dispersers of fruiting tree species.

Digestive performance in neonatal Southern Pacific Rattlesnakes (Crotalus oreganus helleri)

Despite significant research on the metabolic characteristics of digestion in adult snakes, the digestive performance of neonatal snakes is poorly characterized. We examined the energetic costs associated with digestion and the energetic profit derived from the first meal eaten by neonatal Southern Pacific Rattlesnakes ( Crotalus oreganus helleri Meek, 1905). Composition of venom of C. o. helleri changes through ontogeny, becoming richer in proteolytic components that are hypothesized by some to enhance the rate of digestion. Therefore, we also investigated whether venom type (proteolytic-component-rich “adult” versus neurotoxin-rich “neonate” venom) affected digestive performance. We examined specific dynamic action (SDA), apparent assimilation efficiency, rate of digestion, and gut passage time in snakes fed prey killed with either “adult” or “neonate” venom at 22 and 30 °C. Although digestion progressed more quickly at 30 °C compared with 22 °C, there were no significant differences in digestion rate or assimilation efficiency owing to venom type; however, our statistical power was limited by small sample size. Despite the lack of “digestive experience”, the apparent assimilation efficiency was remarkably high (mean of 92%) and greater than published values for Crotalinae species. Based on these results, we hypothesize that neonatal C. o. helleri make a big energetic profit from their first meal by digesting efficiently and economically.

Digestive efficiency of Knysna and purple-crested turacos fed varying concentrations of equicaloric and equimolar artificial fruit

Avian frugivores have been somewhat poorly studied with regards to the effects that different fruit sugar types and concentrations have on their digestive efficiencies. Therefore, two relatively large South African frugivores, the Knysna turaco (Tauraco corythaix) and the purple-crested turaco (Gallirex porphyreolophus), were fed artificial fruit that contained equicaloric and equimolar concentrations of different sugars, to determine their daily food and energy intake, digestive efficiencies and digestive transit times. The artificial fruit contained 6.6, 12.4 or 22%, or 0.42, 0.83 or 1.66 mol l(-1) sucrose or glucose. Food intake of both turaco species increased with decreasing sugar concentration and molarity, irrespective of sugar type, suggesting compensatory mechanisms for energy requirements. Apparent assimilation efficiencies of both turaco species ranged from 61.4-90.0% and 60.2-92.4% for equicaloric and equimolar artificial fruit diets, respectively. Digestive transit times for both turaco species were slowed with an increase in sugar concentration and molarity, irrespective of sugar type. Consequently these two frugivores appear to be tolerant of sugar type and would be expected to select fruits based on energy yields. Future studies of the composition of indigenous forest fruit sugars may give insight into food preferences of the turaco species and their role as potential seed dispersers of fruiting tree species.

Sugar preferences and digestion by Cape white-eyes, Zosterops virens, fed artificial fruit diets

Sugar preferences and digestion of sugars in artificial fruit of different sugar types and concentrations were investigated in a South African facultative frugivore, the Cape white-eye (Zosterops virens). We studied sugar preferences, daily food and energy intake, and digestive efficiencies and transit times with birds that were fed artificial fruit diets comprising 6.6, 12.4 and 22% glucose and sucrose, respectively. Digestive transit rates of birds fed artificial fruit diets were faster for glucose compared with sucrose diets, irrespective of concentration. Birds showed increased food intake with decreasing glucose concentration, but showed no significant differences in these intake amounts with differing sucrose concentrations. Apparent assimilation efficiencies were lower than most nectar studies, and were generally higher for glucose fruit (79.3–85.6%) compared with sucrose fruit (69.0–78.4%). In choice tests, birds preferred sucrose over glucose of equivalent weight, irrespective of concentration. It appears likely that the Cape white-eye has high levels of sucrase activity, although further study is needed to verify this.

Sugar Preferences and Digestion by Cape White-Eyes,Zosterops virens, Fed Artificial Fruit Diets

Sugar preferences and digestion of sugars in artificial fruit of different sugar types and concentrations were investigated in a South African facultative frugivore, the Cape white-eye (Zosterops virens). We studied sugar preferences, daily food and energy intake, and digestive efficiencies and transit times with birds that were fed artificial fruit diets comprising 6.6, 12.4 and 22% glucose and sucrose, respectively. Digestive transit rates of birds fed artificial fruit diets were faster for glucose compared with sucrose diets, irrespective of concentration. Birds showed increased food intake with decreasing glucose concentration, but showed no significant differences in these intake amounts with differing sucrose concentrations. Apparent assimilation efficiencies were lower than most nectar studies, and were generally higher for glucose fruit (79.3–85.6%) compared with sucrose fruit (69.0–78.4%). In choice tests, birds preferred sucrose over glucose of equivalent weight, irrespective of concentration. It appears likely that the Cape white-eye has high levels of sucrase activity, although further study is needed to verify this.

Specific dynamic action, apparent assimilation efficiency, and digestive rate in an arboreal pitviper,Trimeresurus stejnegeri stejnegeri

The major aims of this study were to assess the effects of temperature (15–35 °C) and meal size (less than 30% snake mass) on the metabolism and digestive physiology of Trimeresurus stejnegeri stejnegeri Schmidt, 1925 and to compare the results with those of terrestrial snakes. Specific dynamic action (SDA), peak Vo2, and scope of peak Vo2increased with meal size. Temperature had little effect on SDA. With regression analysis, we found that baseline metabolic rates of T. s. stejnegeri were generally smaller than that for frequent feeders and larger than that for infrequent feeders. We generalized three types of SDA profiles among T. s. stejnegeri and terrestrial snakes (including frequent and infrequent feeders). Trimeresurus stejnegeri stejnegeri had a more shallow and extended profile of postprandial metabolism, which did not support our hypothesis that the pace of digestive metabolism of arboreal snakes is faster than that of terrestrial snakes. The apparent assimilation efficiency (range 0.698–0.884) was significantly lower at 15 °C. At the preferred temperature (26.5–28.8 °C) for a postprandial 50 g snake, the simulated ratios of four parameters (time to peak Vo2, first defecation time, gastric digestion time, and final defecation time) to SDA duration approximated 0.2, 0.5–0.7, 0.7, and 1.1, respectively.

Effects of meal size, clutch, and metabolism on the energy efficiencies of juvenile Burmese pythons, Python molurus

We explored meal size and clutch ( i.e., genetic) effects on the relative proportion of ingested energy that is absorbed by the gut (apparent digestive efficiency), becomes available for metabolism and growth (apparent assimilation efficiency), and is used for growth (production efficiency) for juvenile Burmese pythons ( Python molurus). Sibling pythons were fed rodent meals equaling 15%, 25%, and 35% of their body mass and individuals from five different clutches were fed rodent meals equaling 25% of their body mass. For each of 11–12 consecutive feeding trials, python body mass was recorded and feces and urate of each snake was collected, dried, and weighed. Energy contents of meals (mice and rats), feces, urate, and pythons were determined using bomb calorimetry. For siblings fed three different meal sizes, growth rate increased with larger meals, but there was no significant variation among the meal sizes for any of the calculated energy efficiencies. Among the three meal sizes, apparent digestive efficiency, apparent assimilation efficiency, and production efficiency averaged 91.0%, 84.7%, and 40.7%, respectively. In contrast, each of these energy efficiencies varied significantly among the five different clutches. Among these clutches production efficiency was negatively correlated with standard metabolic rate (SMR). Clutches containing individuals with low SMR were therefore able to allocate more of ingested energy into growth.

Prey envenomation does not improve digestive performance in western diamondback rattlesnakes (Crotalus atrox)

Although the toxic properties of snake venoms have been recognized throughout history, very little is known about the adaptive significance of these powerful mixtures. This study examined the popular hypothesis that prey envenomation enhances digestion by influencing the energetic costs of digestion and assimilation, gut passage time, and apparent assimilation efficiency (ASSIM) in western diamondback rattlesnakes (Crotalus atrox), a species whose venom is recognized for its comparatively high proteolytic activities. A complete randomized block design allowed repeated measures of specific dynamic action and gut passage time to be measured in eight snakes ingesting four feeding treatments (i.e., artificially envenomated live mice, artificially envenomated prekilled mice, saline injected live mice, and saline injected prekilled mice). A second experiment measured ASSIM in eight snakes ingesting a series of six artificially envenomated or six saline injected mice meals over an 8-week period. Contrary to expectations, the results of both these experiments revealed that envenomation had no significant influence on any of the measured digestive performance variables. Gut passage time averaged 6 days and ASSIM averaged 79.1%. Twenty-one hours following ingestion, postprandial metabolic rates exhibited factorial increases that averaged 3.9-fold greater than resting metabolic rate. Specific dynamic action lasted on average 88 hr and accounted for 26% of the total ingested energy. The results of this study reinforce the need to systematically examine the potential adaptive advantages that venoms confer on the snakes that produce them.

Sugar preferences, absorption efficiency and water influx in a Neotropical nectarivorous passerine, the Bananaquit ( Coereba flaveola)

Nectarivory has evolved repeatedly in a number of unrelated bird taxa throughout the world and nectar feeding birds, regardless of their taxonomic affiliation, display convergent foraging and food processing adaptations that allow them to subsist on weak sugar solutions. However, phylogeny influences sugar type preferences of nectarivores. We investigated sugar preferences, assimilation efficiency and water flux in a Neotropical honeycreeper, the Bananaquit ( Coereba flaveola; Coerebidae), a member of a radiation of tanagers and finches. Bananaquits showed no preference for nearly equicaloric (25% w/v) sucrose, glucose, fructose or glucose–fructose mixtures in pair-wise choice tests. In agreement with this lack of preference, they were equally efficient at absorbing sucrose and both hexoses. Apparent assimilation efficiency of these sugars was around 97.5%. In pair-wise tests, Bananaquits displayed a strong preference for the most concentrated sucrose solution when the lowest concentration ranged from 276 to 522 mM. Between 522 and 1120 mM sucrose solution concentrations, Bananaquits were able to adjust their volumetric food intake in order to maintain a constant energy intake. At solution concentration of 276 mM, birds could not maintain their rate of energy intake by increasing food consumption enough. We consider that at low sugar concentrations, Bananaquits faced a physiological constraint; they were unable to process food at a fast enough rate to meet their energy needs. We also explored the possibility that dilute nectars might be essential to sustain high water needs of Bananaquits by allowing them to control osmolarity of the food. Between 276 and 1120 mM sucrose solution concentrations, average amount of free water drunk by Bananaquits was independent of food concentration. They drank very little supplementary water and did not effectively dilute concentrated nectars. The evidence suggests that water bulk of dilute nectars is a burden to Bananaquits.